1. Field of the Invention
This invention relates to a technique for curing of photo-set resins in resin/fiber matrix composites, and particularly relates to a technique for extremely rapid, precisely controlled deep curing of resin/fiber composite parts, both during and after lay-up, using an excimer laser in seamless overlapping scans as a source of ultraviolet curing radiation.
2. Description of Related Art
In the manufacturing of thermoset and photo-set polymer-matrix composite parts, curing is the key process step that transforms the molecular structure of the composite material, stabilizing it in the desired shape. The polymer-matrix composite materials, known as resin matrix composites, include a base polymeric material that encapsulates reinforcing fibers. These composite structures are generally classified as thermoplastic resin or thermoset resin. Thermoset resin is preferred for many composite parts since they are "set" irreversibly (that is, the long-chain molecules of the polymer become cross-linked in a permanent three-dimensional arrangement) whether by heat, room-temperature chemical bonding or by ultraviolet or other photo-setting radiation. Unlike a thermoplastic resin part that can be melted down and re-shaped, the thermoset part configuration is not reversible; the resulting composite part, once cured, is very durable even when subjected to heat and chemical stress.
The reinforcing fiber may consist of glass, carbon, boron, aramid materials or other fiber, whose function is to add strength and stiffness. Many of these fiber materials are selected because of their light weight and high stiffness. These properties are especially desirable for parts in such structures as aircraft wings, automotive body panels, and high-end sporting goods. The products can range in size, from a few square inches in area for some molded parts, to tens or even hundreds of square feet for aerospace structures. The products also have a range of thickness, and are capable of achieving highly complex geometric forms including tapers and integral reinforcing structures.
An important element in the manufacturing of composite structures is a curing system that can handle large and complex parts with a high processing throughput. The curing process determines not only the ultimate performance of the product (by setting the strength of the adhesion between the fiber and the matrix and determining the final shape of the product after shrinkage) but also the economics of the entire manufacturing process through such key factors as materials cost, process cost, throughput, and yield. Throughput is usually measured as the number of units of production per day, and yield is usually measured as a percentage of the number of acceptable units per hundred units manufactured.
In many of these applications, the curing process can take several hours to complete. Furthermore, thermal stresses may arise in the composite parts during the curing operation, due to the different expansion characteristics of the composite materials and the tooling, and due to configuration complexities of the composite structures themselves. These thermal stresses may cause the part to shrink unevenly, warp, and/or retain unwanted residual stresses. Additionally, the curing process in thermoset resins initiates an exothermic reaction in the composite part, which complicates the process control. In an attempt to control the properties of the cured part and minimize the cure time, manufacturers have developed empirical `recipes,` process models, and sensor-based control systems. These methods are somewhat successful, but they are also expensive due to the trial and error process and the highly customized tooling that is required. When fabricating large structures, it is difficult to control directly the amount and rate of heat applied to the resin. It is therefore desirable to develop a large-volume, high-throughput curing system that can provide manufacturers with direct control of the curing process.
Designers of UV-based curing systems in the prior art have generally not considered how to deliver the ultraviolet radiation to initiate the curing process. The most precise controls of ultraviolet radiation have been developed for microelectronics manufacturing in which precise patterns are imaged onto photoresists for patterning microelectronics products such as integrated circuits and circuit boards. This patent presents a novel, UV-based, large-area scanning system for photothermal processing of composite structures. It is based on a scan-and-repeat exposure technology that allows seamless curing of large composite structures by delivering UV radiation in a concentrated controlled beam.